World's End by Richard Jefferies

He happened to light on a volume of notes made upon the remarkable properties of gases. They began with a description of those curious caves and even meadows abroad wherein no animal can live, and the place is strewn with the bones of birds and beasts who have incautiously entered the infected circle. This effect was proved to proceed from a vapour which rose up from the earth, and was no sooner breathed than it produced asphyxiation. Animals who inhaled it fell as if shot, so sudden was the action.

Gases as deleterious were often generated in wells, caves, and confined places, such as the huge tuns or casks used by brewers to keep their beer in, and which had often proved fatal to the men who attempted to scour them out.